While not having a lot of experience with transmissions, YET. I will try and post what I know, and have read.
You can go to just about any 4x4 page and get the stats for just about any transmission. Terry Howe's page is about the best.
If you have a SHORT vehicle, such as a CJ-5. Then the Borg-Warner T-18 (CJ-version) is just the ticket.
This baby is short, and strong. And MOST importantly, has the 6.32 granny gear. Not to mention the 7.44 reverse.
And the adaptor to the Dana 20 t-case is about 1". So overall length is great. Around 12 3/4".
And while, for now, the Dana 20 only has a low range of 2.03, there are a few companies that
are making lower ratios.
Tera Mfg (Tera Flex) is making a kit with 3.125 that is a drop in.
There are other who are working on a kit with 3.7 low, but with some grinding to remove the internal flashing.

The T-18 has only one real drawback. GETTING ONE. It is a difficult task to find a GOOD condition short version T-18
that someone wants to sell for a 'reasonable' price. Of course who would want to sell a good one?
Other options is to get the Ford 2WD version, much longer. And get the kit that replaces the input shaft.
Or get your hands on one that just isn't in THAT good of shape.
Usually it is a fairly easy job to get your hands on one that has had the main gear cluster ground up, I got one like this.
Then rebuild it. This isn't a job for most do-it-your-selfers!!! Putting it all back with the correct tolerances is a
paramount point in this rebuild. But the money saved will make it easier to afford the new parts.
I recommend getting a tranny that needs the rebuild. Rebuilding it, or having a shop do it (do a lot of
research on who is good in your area first), and then you can the TOTAL piece of mind that your 'new'
T-18 will go for the long haul.